The present invention relates, in general, to power semiconductor devices and methods of forming such devices. More particularly, the present invention relates to alloyed drain field effect transistors (ADFET's) and methods of forming such devices.
Power semiconductor devices are used in such applications as variable speed motor controllers, uninterruptible power supplies, and high frequency welders. Included in this category of semiconductor devices have been power metal oxide semiconductor field effect transistors (MOSFET's), insulated gate bipolar transistors (IGBT's) and Schottky Injection field effect transistors (SINFET's). All three types of devices offer similar gate drive capabilities, as well as a wide safe operating area (SOA). However, they differ most significantly in their conduction characteristics and switching speeds, IGBT's having the best conduction characteristics and power MOSFET's having the fastest switching speeds. The performance of a SINFET is close to that of a MOSFET being slightly slower while having slightly better conduction characteristics. Therefore the selection of a device for a specific application, requires a trade-off between switching speed and conduction characteristics. New devices, such as the ADFET, improve upon the switching speed performance of the IGBT while maintaining its conduction characteristics, thus reducing the trade-off costs associated with the prior art devices.
Typically, IGBT's include a substrate layer of a P conductivity type on which a relatively lightly doped epitaxial layer of an N conductivity type is formed, thereby forming a PN junction. Most of the IGBT device structure is fabricated in the epitaxial layer (more commonly referred to as the drift region), wherein the substrate layer serves as a bottom-side contact for the IGBT and forms an emitter region of a PNP transistor. The lightly doped epitaxial layer produces a drift region having a low conductivity, a high resistivity, and is capable of supporting high voltages. However, the high resistivity of the drift region increases the "on" resistance, which in turn limits the current rating of the IGBT. The PN junction formed between the substrate and the epitaxial layer lowers the "on" resistance by injecting minority carriers into the drift region. In addition, this injection of minority carriers increases the conductivity of the drift region.
The modulation of the "on" resistance and the conductivity of the drift region by injection of minority carriers across the PN junction implies that both majority and minority carriers participate significantly in current flow in the IGBT. Although the use of both carrier types is advantageous for "on" resistance and conductivity, during turn-off of the IGBT, the carriers produce a "tail" current which delays the turn-off of the device. One solution to reducing this "tail" is to insert a buffer zone between the substrate and the drift region, wherein the buffer zone is epitaxial silicon having the same conductivity type as the drift region but having a higher concentration of impurity material. However, this solution requires the formation of a second epitaxial layer, in addition to the first layer or drift region. A second solution is based upon the belief that the charge associated with the "tail" current may be reduced and the turn-off delay improved by, for example, providing recombination centers in the lattice structure of the epitaxial layer. These recombination centers may be formed by creating imperfections or damage in the epitaxial layer lattice structure using such means as irradiating the epitaxial layer. The SINFET provides still a third solution, control of the minority carrier injection into the drift region. Thus the formation of a Schottky barrier drain contact provides an efficient conduction path for electrons during turn off, thus improving the switching speed. A SINFET providing such improved performance is the subject of U.S. Pat. No. 5,397,716 issued to Samuel J. Anderson and assigned to the same assignee as the present invention.
However while SINFET's, for example the device in U.S. Pat. No. 5,397,716, offer a significant improvement in switching speed over IGBT's, the improvement is at the cost of conduction characteristics. Accordingly, it would be advantageous to form a device with performance that exceeds that of a SINFET. Such a device would have an "on" resistance close to that of an IGBT, as well as switching speeds close to that of a power MOSFET. It would be further advantageous that the method of forming this device exclude the formation of recombination centers and a buffer zone as these steps increase the cost and cycle time associated with manufacturing IGBT's.